The simian virus 40 (SV40) small-t antigen enhances viral transformation efficiency, presumably through its reported effects on cellular growth induction. The recent finding that small-t binds the cellular enzyme, protein phosphatase 2A (PP2A), reducing its activity, provides an important clue to the action of small-t. In the proposed experiments, we will: (1) use defined point mutants to probe the interaction of small-t and PP2A, with particular emphasis on the relationship between PP2A inhibition and the enhancement of transformation, (2) determine sequences of small-t required for interaction with PP2A, (3) define the structure of small-t and the role of metal binding in maintaining structural integrity and on PP2A inhibition, (4) analyze the phosphorylation state of key proteins within infected cells, notably proteins such as the Na+/H+-antiporter, large-T antigen and others known to play a role in growth promotion, (5) express small-t in Saccharomyces cerevesiae to determine whether this affects yeast cell cycle progression, which is well-defined genetically, and (6) study the effects of small-t on intracellular pH control which may result either by activation of the Na+/H+-antiporter, possibly through phosphorylation, or activation of an alternate H+-extrusion pathway.